The present invention relates to a process for purifying gaseous mixtures. Moreover, the present invention applies to the treatment of liquid hydrocarbon mixtures containing acidic compounds and other sulphur-containing compounds such as the mercaptans and COS. The present invention further relates to an absorbent solution for the implementation of this process.
The invention in particular finds its application in the oil and gas industry. The invention applies in particular to the removal of acidic gases from natural gases. In fact, acidic natural gases represent about 40% of the remaining gas reserves in the world and the specifications relating to the sulphur content of these gases are more and more serious.
The purification of gaseous mixtures, and in particular of hydrocarbon gaseous mixtures such as natural gas, which consists primarily of methane, or of synthesis gas, in order to remove contaminants and impurities from them is a common operation in industry. These impurities and contaminants are in particular the “acidic gases” such as for example carbon dioxide (CO2) and hydrogen sulphide (H2S); sulphur-containing compounds other than hydrogen sulphide (H2S) such as for example carbonyl sulphide (COS) and the mercaptans (R—SH, where R is an alkyl group); water, and certain hydrocarbons. Carbon dioxide and hydrogen sulphide can represent a large proportion of the gaseous mixture from a natural-gas deposit, typically from 3% to 70% by volume, whereas COS is present in much smaller quantities, typically ranging from 1 to 100 ppm by volume, and the mercaptans are present at a content generally below 1000 ppm by volume, for example at a content comprised between 5 ppm by volume and 500 ppm by volume.
Contaminants that must be removed also include the mercaptans. The total quantity of mercaptans in a gaseous mixture originating from a natural gas production site can represent several hundreds of ppm by volume. The two principal mercaptans involved are methyl mercaptan and ethyl mercaptan, but other mercaptans, and in particular the molecules of the type C3SH to C6SH can be present, generally at lower concentration.
The natural gas obtained from a deposit thus undergoes several treatments in order to comply with specifications which are in particular dictated by commercial constraints, transport constraints or constraints connected with safety. These treatments are in particular treatments of deacidification, dehydration, and gasoline stripping. This last-mentioned treatment consists of separating ethane, propane, butane and gasolines, forming liquefied petroleum gas (“LPG”), from the methane gas which is sent to the distribution system.
The specifications on the content of acidic gases in the treated gas (originating from the treatment) are specific to each of the considered products. Thus, the contents of a few ppm are imposed for H2S or the other sulphur-containing compounds, whereas the specifications for CO2 are comprised between several ppm if a liquefaction step for producing liquefied natural gas is provided downstream of the purification process, and a few percent, generally 2%. Similar problems are encountered for the deacidification of liquid hydrocarbon mixtures and in particular liquid hydrocarbon cuts, obtained in the steps of gasoline stripping natural gas, or in the fractionation of crudes in the applications connected with petroleum refining or other of liquid hydrocarbons mixtures.
Numerous processes currently exist for successively or simultaneously removing the acidic gases and other sulphur-containing compounds from gaseous mixtures. Taking into account the specific properties of each of the products to be removed, these processes generally comprise several different successive steps in order to reach all the requirements of the specifications.
Thus, carbon dioxide CO2 and hydrogen sulphide H2S are generally removed in a step using a water-alkanolamine absorbent solution, whereas the other sulphur-containing compounds, such as the mercaptans or COS, are removed during steps using dedicated processes such as an absorption on a molecular sieve or a scrubbing with soda. The water-alkanolamine absorbent solutions were optimized in the past during the development of processes for the removal of hydrogen sulphide and carbon dioxide. These processes are now well known and widely used.
Initially, primary or secondary alkanolamines were used for the treatment of laden gaseous or liquid hydrocarbon streams with acidic gases. Among these alkanolamines, there may be mentioned in particular: MonoEthanolAmine (MEA, 2-aminoethanol), DiEthanolAmine (DEA, N,N-bis(2-hydroxyethyl)amine), DilsoPropanolAmine (DIPA, N,N-bis(2-hydroxypropyl)amine) and 2-aminoethoxyethanol (AEE).
Energy constraints as well as constraints connected with the processes then led the operators to improve these processes, by optimizing the absorbent solutions, for example by using MethylDiethanolAmine (MDEA) for the selective removal of H2S, then mixtures of MDEA and primary or secondary alkanolamines for controlling the quantity of CO2 absorbed. These optimized absorbent solutions can in particular comprise a tertiary amine and an activator.
Examples of these absorbent solutions are given in the following documents:
U.S. Patent Publication No. 2008/0025893 relates to a process for removing CO2 from a gaseous stream, in which the said gaseous stream is contacted with a liquid absorbent selected from an aqueous solution of an amino compound with at least two tertiary amine groups in their molecule and an activator selected from the primary and secondary amines; or an aqueous solution of a tertiary aliphatic amine and an activator selected from 3-methylaminopropylamine, piperazine, 2-methylpiperazine, N-methylpiperazine (NMP), homopiperazine, piperidine, and morpholine (MO).
U.S. Pat. No. 7,374,734 (WO-A1-2004/071624) describes a process for removing the acidic gases from a fluid, in which the said fluid is contacted with an aqueous absorbent comprising at least one tertiary alkanolamine in particular selected from methyldiethanolamine (MDEA), triethanolamine (TEA), ethyldiethanolamine or diethanolamine (DEA); an amine selected from hydroxyethylpiperazine (HEP) or a mixture of the latter with bis(hydroxyethylpiperazine); and piperazine.
U.S. Pat. No. 6,852,144 relates to a process for selectively removing COS relative to CO2 in a stream of a hydrocarbon fluid which contains CO2 and COS, in which the hydrocarbon fluid is contacted with a scrubbing liquid constituted by an aqueous amine solution containing an aliphatic alkanolamine having from 2 to 12 carbon atoms, preferably a tertiary alkanolamine such as methyldiethanolamine (MDEA) or triethanolamine (TEA); and an activator selected from piperazine, methyl piperazine, and morpholine (MO).
U.S. Pat. Nos. 5,209,914, 5,277,885 and 5,348,714 relate to a process of CO2 absorption in a gas, in which the said gas is contacted with an absorbent liquid, which is generally in the form of an aqueous solution of one or more alkanolamines such as MDEA and an activator which improves the absorption of CO2 by the said alkanolamines.
The activator can in particular be selected from the polyamines such as dipropylenetriamine (DPTA), diethylenetriamine (DETA), triethylenetetramine (TETA) and tetraethylenepentamine (TEPA); aminoethylethanolamine (AEEA), hexamethylenediamine (HMDA), dimethylaminopropylamine (DMAPA) and diaminocyclohexane (DACH). Other suitable compounds as activators are methoxypropylamine (MOPA), ethoxypropylamine, aminoethyl piperazine (AEPZ), aminopropyl piperazine, aminoethyl piperidine (AEPD), aminopropyl piperidine, furfurylamine (FA), and ethylmonoethanolamine (EMEA). However, the optimized absorbent solutions are always constituted by water-alkanolamine mixtures.
The processes which use absorbent solutions constituted by water-alkanolamine mixtures have the drawback to be limited to the absorption of carbon dioxide and hydrogen sulphide, and their performance as regards to the removal of the other sulphur-containing compounds such as the mercaptans remains minimal. The other sulphur-containing compounds must therefore be removed in additional steps. For example, a step using molecular sieves makes it possible to remove the mercaptans.
In order to simplify the treatment of gaseous mixtures containing acidic gases, another solution therefore consists of using a hybrid solvent, composed of water, alkanolamine, and an organic compound allowing to solubilize in the solvent, the sulphur-containing compounds, such as the mercaptans. The aim of the processes which use these hybrid solvents, is to remove simultaneously of CO2, H2S, and the other sulphur-containing compounds, such as the mercaptans and COS. These processes use for example sulpholane or methanol as organic compound performing the role of cosolvent.
However, it should be noted, that these processes suffer from a severe penalty owing to the nature of the retained organic compound. In fact, sulpholane leads to considerable coabsorption of hydrocarbons, which causes many problems both with regard to the operation of the sulphur production plants located downstream, and with regard to the quality of the products obtained from these plants. The application of methanol as cosolvent leads to significant losses of alcohol.
Another process for the simultaneous removal of the acidic gases and mercaptans was the subject of the application WO-A1-2007/083012 (FR-A1-2896244), which describes a process for purifying a gaseous mixture containing acidic gases comprising a step of contacting said gaseous mixture with an absorbent solution comprising an alkanolamine, a C2-C4 thioalkanol, and water. The preferred alkanolamine is diethanolamine (DEA), and the preferred thiodiglycol is thiodiglycol (TDG). However, the energy efficiency of the process described in the document WO-A1-2007/083012 (FR-A1-2896244) is still insufficient and needs to be improved. Moreover, this process cannot be adapted easily to the composition of the treated feedstock and/or the specifications to be met as regards to the contents of each of the compounds to be removed. In other words, its flexibility is low.
Therefore, in view of the foregoing, there is a need for a process which provides, in a single step, a simultaneous removal of the acidic gases and other sulphur-containing compounds from a gaseous mixture, which ensures an energy saving relative to the processes of the prior art such as the process described in document WO-A1-2007/083012, and which can easily be adapted so that it is suitable for treating gaseous mixtures of various compositions and complying a wide range of specifications as to the final contents of acidic gases and other sulphur-containing compounds to be obtained. The aim of the present invention is to provide a process for purifying a gaseous mixture containing acidic gases which meets these needs, among others. The aim of the present invention is also to provide such a process which does not have the drawbacks, shortcomings, limitations and disadvantages of the processes of the prior art, and which provides a solution to the problems of the processes of the prior art.